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72 SMT Magazine • March 2017 the amount of solder paste has to be compat- ible with the final required soldering amount. In theory, the smaller the SMD component is, the thicker the stencil has to be. However, keep in mind that the thinner the solder paste is, the more difficult it is to control the tin amount. Basically, the thickness of an ordinary stencil is within the range from 0.12 mm to 0.15 mm. When it comes to fine pitch components (0201 or 01005), a stencil with thickness of 0.1 mm below is needed. Silkscreen Printing Parameter Setting and Modification a. Scraping blade pressure: A slight modifi- cation of scraping blade pressure leads to tre- mendous influence on solder paste printing. If blade pressure is too low, the solder paste will fail to fall at the bottom of stencil aperture and to be effectively transferred to the pad. If blade pressure is too high, the solder paste will be too thin or the stencils will even be damaged. The optimal condition is that the solder paste is scrapped from the surface of stencil totally. b. Printing thickness: Printing thickness largely relies on the thickness of the stencil. A slight modification of solder paste printing thickness can be obtained through blade speed modification and blade pressure. Suitable re- duction of printing speed of blade also leads to increasing of solder paste amount on PCB. c. Stencil cleaning: In solder paste print- ing, the stencil should be cleaned after every 10 units of PCBs have been successfully print- ed in order to eliminate deposits at the bottom of stencils and pervasive solder paste. General- ly, alcohol without water is applied as a clean- ing agent. To obtain genuinely high SMT quality, in- vestigation and analysis have to be carried out on each manufacturing link and key elements so that effective control methods can be cap- tured. In the SMT assembly process, solder paste printing is the most critical. As long as reason- able parameters are set and corresponding laws between them are mastered, high-quality solder paste printing can be achieved. SMT Dora Yang is a technical en- gineer from PCBCart, a China- based prototype and middle-vol- ume PCB assembly service pro- vider. EVALUATING A PCB ASSEMBLER Didn't make it to IPC APEX EXPO 2017? We've got it covered for you. During IPC APEX EXPO 2017, we at I-Con- nect007 featured a state-of-the art micro studio, where over 70 RealTimewith…IPC interviews were conducted. See the opening ceremony, hear from suppli- ers on their new products, learn more about the HDP Users Group, and don't miss the videos by IPC President John Mitchell and his staff. We also created a time-lapse video, which is made up of more than 50,000 photos taken from the event setup to closing. We are sure you will find this very entertaining. The video is about sev- en minutes in length…which is not bad consider- ing you're watching a three-day event! So, get all of your post-show coverage, includ- ing in-depth interviews and the event photo gal- lery, by visiting our RealTimewith…IPC APEX EXPO 2017 site now! IPC APEX EXPO 2017 Coverage with I-Connect007

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